The invention relates to an MR method (MR=Magnetic resonance) for exciting the nuclear magnetization in a limited volume of an object to be examined, utilizing a microcoil which is present in said volume and is subject to at least one RF pulse. The invention also relates to a device for carrying out the method disclosed in claim 1 and to a preferably medical instrument for use in conjunction with the method and with the device.
A method and a device of this kind are known from EP-A 928 927. Therein, a capacitance is connected parallel to the microcoil so that a resonant circuit is obtained which is tuned essentially to the frequency of the RF pulses (RF=Radio Frequency). The (external) magnetic field produced by the RF pulses and traversing the object to be examined is intensified in the close range of the microcoil and its phase is shifted. As a result, the excitation of the nuclear magnetization is intensified in this close range, so that in the MR image this range is highlighted relative to the other parts of the object to be examined.
The excitation of the nuclear magnetization in a limited volume can be utilized in various ways; one possibility consists in forming an MR image of this strictly limited volume, for example a blood vessel. Restricting the MR image to a small volume results in short measuring times and enables fluoroscopic applications. Another possibility consists in the localization of the microcoil and possibly an instrument connected thereto. The instrument, for example a medical instrument at the location to be marked is then provided with a microcoil.
The acquisition of the MR signals for the imaging of a limited volume or for the localization of the microcoil can be performed in a comparatively simple and fast manner. However, MR signals are then received from the entire volume in which the conditions for magnetic resonance are satisfied. This volume is substantially larger than the close range in which the magnetic field of the microcoil is essentially concentrated. Consequently, aliasing artefacts occur in the MR images of the close range; in the case of localization by means of three orthogonal projection measurements, this results in a signal background which could make localization impossible.
Therefore, it is an object of the invention to conceive a method of the kind set forth in such a manner that the excitation of the nuclear magnetization is better restricted to the close range of the microcoil. It is a further object to provide an MR apparatus for carrying out the method as well as an instrument which is especially intended for use in conjunction with the method and the device.
This object is achieved by a method which includes the following steps: generating an RF pulse with a frequency spectrum which does not overlap the Larmor frequency, so that the nuclear magnetization in the object to be examined is not excited thereby, generating an additional frequency spectrum by a microcoil under the influence of the RF pulse, which additional frequency spectrum overlaps the Larmor frequency in such a manner that the nuclear magnetization is excited in a prescribed range, referred to herein as the close range, of the microcoil within a limited volume of the overall volume, as further discussed herein.
According to one embodiment of the invention, nuclear magnetization beyond the prescribed range of the microcoil cannot be excited. This occurs because the conditions for magnetic resonance are not satisfied beyond the prescribed range due to the specific spectrum of the RF pulse modified according to the invention. Due to the modification of the spectrum by the microcoil, the conditions for magnetic resonance are satisfied only in the prescribed range (i.e., close range) of the microcoil. Consequently, nuclear magnetization can be excited only in the close range. The invention thus offers the advantage that the MR signals practically do no contain any background signal originating from the overall volume, so that the problems of the method known thus far are avoided because of the essentially higher signal contrast.
In one embodiment, the invention discloses an MR apparatus which is suitable for carrying out the method according to the invention. Another embodiment describes a medical instrument which is particularly suitable for use in conjunction with a method or a device according to the invention. In the latter embodiment, the medical instrument includes a microcoil which is intended for localization in an object to be examined, the microcoil being wired so as to form a non-linear resonant circuit.
Additional embodiments disclose various possibilities for the acquisition of information from the spatially limited excitation of the nuclear magnetization. For example, one method is disclosed wherein the nuclear magnetization is excited alternately in the close range and in a larger volume enclosing said close range. As a result, additionally MR signals are acquired for an MR image in which the position resulting from the localization can be visualized, if desired. The alternating acquisition of MR signals for imaging and for localization is advantageous notably in the case of moving objects, because the imaging and the localization relate to practically the same time interval. In order to enhance the motional resolution, the duration and the frequency of the two operations (that is, the in duty cycle) can be advantageously adapted to one another.
According to one embodiment, anti-parallel connected diodes provide means which non-linearly co-operate with the microcoil. The microcoil couples with the anti-parallel connected diodes in such a manner so as to form a non-linear resonant circuit. Alternatively, a miniaturized circuit generates a signal having the Larmor frequency in response to a reception of signals having a frequency beyond the Larmor frequency.